1. Field of the Invention
The present invention relates to a method and apparatus for controlling combustion of a furnace and/or a burner using an oxygen sensor.
2. Description of Related Art
The general combustion control methods for a furnace and/or a regenerative combustion system are the following:
(1) a method wherein fuel and air are supplied and cut off by operating solenoid valves installed in the fuel system and the air supply system, PA1 (2) a method wherein supply amounts of fuel and air are controlled by the respective pressure control valves installed in respective systems which are correlated with each other in operation, and PA1 (3) a method wherein the pressure control valves are replaced by flow control valves in the above method.
In the above, the regenerative combustion system is a system known by, for example, Japanese Patent Publication HEI 4-270819. In that system, high temperature exhaust gas is exhausted through a heat storage member, and most of the heat of the exhaust gas is stored in the heat storage member. When gas exhaust and air supply are switched and supply air passes through the heat storage member, the heat stored in the heat storage member is released to heat the supply air. Due to this, the thermal efficiency of the system is greatly improved.
However, in any of the above-described methods, an attempt to raise accuracy of the control will be accompanied by complication and increase in cost of the system.
To raise accuracy of the control, it would be effective to control an air ratio based on an oxygen concentration of exhaust gas. However, conventional sensors have the problems that they are expensive and it is difficult to find degradation of or damage to the sensor.
Particularly, with the regenerative combustion system, there is a problem that because the air ratio is likely to vary due to (a) blockade of the heat storage member, (b) leakage of supply air to exhaust gas in the air supply and gas exhaust switching mechanism, and/or (c) pressure change accompanying the temperature change, it is difficult to operate the system at an optimum air ratio for a long time period.
Further, control of the air ratio based on the oxygen concentration in the exhaust gas cannot provide inspection of concentrations of unburnt components such as carbon monoxide and hydrocarbons included in the exhaust. Therefore, even if the air ratio is controlled, unburnt components more than an allowable limit may be included in the exhaust gas. To prevent the unburnt components from being exhausted to the atmosphere, it would be necessary to provide some device for detecting the amount of the unburnt components, which will increase the cost of the combustion control system.